The present invention relates to a process and apparatus, used in incremental measuring systems, for the regulation of the keying ratio of an electrical signal.
As is well known, incremental measuring systems produce impulses. The number of the impulses represents a measure of the linear displacement or the angle of rotation of a first object with respect to a second object. The incremental measuring systems often comprise photo-electric components. Such photo-electric digital length or angle measuring systems are used, for example, in processing machines for the measurement of the relative position of machine parts which are moveable with respect to one another. The measuring systems are also used in machines for processing a work piece or in measuring machines for the determination of the position or of the dimensions of a test object.
The resolution of such measuring systems is limited by the grid constant of the precision scales used in the instrument. However, the resolution of the measurement can be improved by interpolation techniques. The accuracy of the interpolation is dependent on the quality of the signal parameters. Faulty signal parameters in the form of amplitude fluctuations, superposed direct voltage components, phase errors and inadmissible harmonic components can severely limit the accuracy of the interpolation. Length and angle measuring systems are known which comprise an arrangement for eliminating such faulty signal parameters.
German unexamined patent specification OS No. 27 29 697 describes a measuring system with an arrangement for correcting the faulty signal parameters of the scanning signals. The deviations of the signal parameters are determined and stored in a memory as digital correction values. The digital correction values are then used by a computer prior to the interpolation to correct the digitized scanning signals. This process requires a storage unit and relatively extensive computing operations.
German patent No. 19 45 206 discloses an arrangement for the interpolation of path or angle dependent electrical signals in which the electrical signals are connected to a series of comparators with different trigger levels. These different trigger levels are adapted to the prescribed forms and amplitudes of the electrical signals. A reference signal, dependent on the amplitude of the electrical signal, is generated which adapts the trigger levels in response to the prescribed interpolation steps, or maintains the amplitude of the electrical signal constant by variable amplification in dependence on the reference signal. In this arrangement, continuously operative at all frequencies of the electrical signal, a correction signal is formed and superposed on the electrical signal or the control input of the comparators.
German unexamined patent specification OS No. 30 46 797 describes an error correction which is produced from a direct current level displacement of the electrical sinusoidal signal of a photo-electric measuring system. A mean value is generated from the maximum and minimum values of the electrical signal. The mean value is then compared in each case with a reference value for the generation of an error signal. The electrical signal is corrected by superposing the error signal with the electrical signal. The correction is dependent upon a further electrical signal which is phase shifted through 90.degree..
German patent No. 24 59 749 describes a pick-up monitoring arrangement for measuring the rate of rotation. The movement dependent electrical signal is converted by means of an integrator into a signal proportional to the keying ratio. The signal is then compared with an upper and lower reference limit. If the signal rises or falls above or below the respective reference limit, an alarm signal is triggered.
German published application AS No. 20 61 588 discloses a circuit for the regulation of a keying ratio of impulse sequences. A low pass component feeds only correction signals below a given frequency to an amplifier and is positioned in the feedback circuit of a closed regulating circuit.
These known processes and systems have the common disadvantage that, after the measuring system recovers from an operating pause, no error signal is present for the regulation of the faulty electrical signal. This disadvantage arises because the electrical components in a regulating circuit can only deliver the correction signal required for the regulation when the electrical components are in the operative state. A further disadvantage is that these known processes and arrangements produce an error signal at all frequencies of the periodic electrical signal. At low frequencies of the periodic electrical signal, the error signal is not operative to sufficiently correct for the influences of temperature, aging or drift. The inoperability arises because the electronic components used in the regulation circuit are only dependably operative within an admissible frequency range.